The usual optical techniques for recording information on storage media is done by ablation of thin films with a focused laser beam. The thin film storage media using laser ablation may have very complicated structures, have low signal-to-noise ratios, require large amounts of laser energy, and suffer from degradation with time. Other optical techniques involve pit formation or bubbles in the thin film and in general require a surface deformation on the thin film to modify the optical properties of the active medium. One feature in common with all optical storage systems is the fact that optical storage systems utilize diffraction limited optics which is approximated by the wavelength of the laser light used to modify the material and to read the stored information from the storage media. As the wavelength of the laser light decreases, the optical spot size gets smaller, thus leading to higher bit density optical storage systems.
In U.S. Pat. No. 5,024,927 which issued on Jun. 18, 1991 to K. Yamada et al., an information recording medium capable of recording and erasing information with the application of electro-magnetic waves is described comprising a recording layer formed on a substrate, the recording layer including a carbon-based material, a polymer prepared by subjecting a pigment to plasma polymerization and an optically reversible material whose optical characteristics can be reversibly changed. The optically reversible material may be finally-divided particles of a metal dispersed in the carbon-based material. The carbon-based material serves as the matrix for the optically reversible material in the recording layer. Specific examples of the optically-reversible materials include chalcogens such as Te and Se, alloys of chalcogens, materials whose crystalline phase is optically changeable, such as Zn-Ag and Cu-Al-Ni, Phthalocyanine-type pigments whose crystalline phase is optically changeable, and organic chalcogen compounds prepared by plasma CVD, such as diphenyl tellurium, diphenyl selenium, dimethyl tellrurium, dimethyl selenium, tellurium diisopropoxy diacetylacetonate and selenium diisopropoxy diacetylacetonate.
In U.S. Pat. No. 4,812,385 which issued on Mar. 14, 1989 to K. C. Pan, a write once read many (WORM) optical memory system is described. A recording laser provides a laser beam through a series of lenses to be focussed as a spot on a rotating disk. The disk has a layer of amorphous thin film material thereon comprising an alloy having a composition within a polygon in a ternary composition diagram of antimony, zinc and tin. Writing is accomplished by heating a location above the transition temperature wherein the amorphous material is converted to a crystalline material. A separate laser is shown for reading data from the rotating disk by detecting the reflectance of the alloy in either the crystalline or amorphous state. The amorphous state is very stable.
It is well known that certain polymers may undergo an irreversible index of refraction change in response to irradiation of ultraviolet light. In U.S. Pat. No. 3,689,264 which issued on Sep. 5, 1972 to E. A. Chandross et al., readily observable irreversible index of refraction changes in poly (methyl methacrylate) sensitized by the addition of ingredients to enable photo-induced cross-linking was described when irradiated with ultraviolet light from a laser.
In U.S. Pat. No. 4,994,347 which issued on Feb. 19, 1991 to W. K. Smothers, a substantially solid, storage stable photopolymerizable composition is described that forms a refractive-index image upon exposure to actinic radiation. The composition consists essentially of: a solvent soluble, thermal plastic polymeric binder; N-vinyl carbazole; and a hexaarylbiimidazole photoinitiator system having a hydrogen donor component.
In U.S. Pat. No. 4,981,777 which issued on Jan. 1, 1991 to M. Kuroiwa et al., a thin optical recording film is described comprising at least one low melting point metal, carbon and hydrogen on a substrate, and heat treating the so formed film on the substrate at a temperature of from 70.degree. to 300.degree. C. for a period of at least 5 seconds. The heat treatment is carried out at a temperature well below the melting point of the low melting point metal contained in the film. It has been found that the recording sensitivity of the recording film can be enhanced by the heat treatment according to the invention. By the term "enhanced recording sensitivity", it is meant reduction in energy of an energy beam such as a laser light required for recording information in unit area of the recording film. The low melting point metal element in the recording film may be tellurium, bismuth, zinc, cadmium, lead and tin used alone or in combination. The carbon content of the recording film is preferably from 5 to 40 atomic percent based on the whole film.
In U.S. Pat. No. 4,647,512 which issued on Mar. 3, 1987 to N. Venkataramanan et al., a plasma assisted chemical vapor transport process is described. The material, diamond-like carbon may be produced by plasma assisted chemical vapor transport (PACVT) process in which hydrogen is employed as the reactive process feedgas and in which the deposition process is conducted in a controllably energetic ion bombardment of the surface on which the film of diamond-like carbon is grown. Further, FIG. 4 of '512 displays the optical transmission of diamond-like carbon films obtained on KBr substrates whose intrinsic transparency is also shown in FIG. 4. The films with a thickness of about 1/2 micrometer exhibit high transparency at UV wavelength. The films exhibit a transparency of more than 50% for wavelengths above about 200 nm and more than 90% above about 400 nm. The films also exhibit a high index of refraction, about 2 at 850 nm.
The use of diamond-like carbon film as a protective coating on magnetic media has been described in U.S. Pat. No. 4,647,494 which issued on Mar. 3, 1987 to B. S. Meyerson et al. and assigned to the assignee herein. The diamond-like carbon layer provided a superior wear-resistant coating over the metallic magnetic recording layers. An intermediate layer of silicon having a minimum thickness of a few atomic layers was formed between the diamond-like carbon protective layer and the metallic magnetic recording layer to provide strong adhesion. The diamond-like carbon layer was plasma deposited.
In U.S. Pat. No. 4,833,031 which issued on May 23, 1989 to H. Kurokawa et al., a protective film was described made of a diamond-like carbon film and an organic compound film over a ferromagnetic metal recording film. The protective film provided excellent durability and small spacing loss and as a result high density magnetic recording was obtainable. The organic film on the amorphous carbon film included an organic compound having at least one fatty alkyl group having at least 8 carbon atoms at the end of a molecular structure thereof.
In a publication by A. Grill et al. entitled, "Bonding, interfacial effects and adhesion in DLC", SPIE, Vol. 969, Diamond Optics (1988), the structure and optical properties of diamond-like carbon (DLC) films are described. Diamond-like carbon films may contain sp.sup.2, sp.sup.3 and even sp.sup.1 coordinated carbon atoms in a disordered network. The ratio between the carbon atoms in the different coordinations of carbon atoms is to a great extent determined by the hydrogen content of the films. Typically, diamond-like carbon layers are seen to be weakly absorbing in the visible spectrum, tending toward transparent in the infrared spectrum. Their transparency makes diamond-like carbon films good candidates as a protective optical coating.
In a publication by V. Y. Armeyev et al. entitled, "Direct laser writing of conductive pathways into diamond-like carbon films", SPIE, Vol. 1352, Laser Surface Microprocessing, pp. 200, (1989), microprocessing of diamond-like carbon films with a continuous wave argon laser at 488 nm wavelength was described. Conductive lines were formed in the amorphous carbon films several micrometers wide having a resistivity of about 4.times.10.sup.-2 .OMEGA.cm. The conductive lines were formed by graphitization as evidenced by Raman spectroscopy. The graphitization temperature threshold lies in the range from 400.degree. to 500.degree. C. The etching threshold where carbon is oxidized is found in the temperature range near 600.degree. C. The use of diamond-like carbon as the active material for once-write optical recording is suggested, if the change in reflectance due to local graphitization is high enough. By using a finely focused He-Ne laser beam at 0.633 nm, the contrast in reflectivity was about 2 for scanning a graphitized spot against the as-deposited film. The graphitized strip was written at a power of 740 milliwatts.
In a publication by S. Prawer et al. entitled, "Pulsed laser treatment of diamond-like carbon films", Appl. Phys. Lett. 48, 1585 (1986), conducting pathways having a resistance of 0.10 .OMEGA.cm were formed in insulating (10.sup.6 .OMEGA.cm) diamond-like carbon film using pulsed laser irradiation at 0.53 micrometer. Below a laser intensity threshold of 0.2 J/cm.sup.2 of a pulsed, 70 ns, neodymium: yttrium aluminum garnet operating at 0.53 micrometers, there was no observable interaction between the laser and the film. Above 0.2 J/cm.sup.2, the diamond-like carbon film was partially graphitized and the effected region displayed a terrace-like structure with sharp edges. Two processes were described resulting from the interaction of the laser with the diamond-like carbon film. The diamond-like carbon film was transformed into a form of graphite for laser intensities exceeding a threshold of about 0.2 J/cm.sup.2 and ablation of the graphite occurs.
In a publication by M. Rothschild et al. entitled, "Excimer-laser etching of diamond and hard carbon films by direct writing and optical projection", J. Vac. Sci. Technol. B, 4, 310 (1986), diamond-like carbon thin films were explored as positive-acting resist for semiconductor patterning. An ArF laser at 193 nm wavelength, was particularly suitable for interaction with diamond-like carbon, since the proton energy at this wavelength 6.4 eV is higher than the bandgap of diamond 5.4 eV. The crystal was highly absorptive. Crystalline diamond and diamond-like carbon thin films were etched with the Excimer laser. Deep structures, about 15 micrometers, were obtained in the direct write configuration and linewidths less than the laser wavelengths were generated in optical projection. The laser-induced etching takes place via surface graphitization, by a combined thermal/photochemical conversion, followed by sublimation and/or reaction.
In a publication by R. J. Gambino et al. entitled, "Spin resonance spectroscopy of amorphous carbon films", Solid State Comm., Vol. 34, pp. 15-18 (1980), printed in Great Britain, amorphous carbon was prepared by the plasma decomposition of propane providing a film which was hard, transparent, insulator much like diamond in its physical properties. The composition of amorphous carbon is described as being a random network of sp.sup.3 and sp.sup.2 bonded carbon with the relative fraction of each depending on the method of preparation and the process parameters.